By way of example, laser plasma sources are used for application in lithography. Thus, for example, the required EUV light is generated by an EUV light source based on a plasma excitation, in respect of which FIG. 14 shows an exemplary conventional setup, during the operation of a projection exposure apparatus configured for the EUV range (e.g. at wavelengths of e.g. approximately 13 nm or approximately 7 nm).
This EUV light source includes a high-energy laser (not shown here), e.g. for generating infrared radiation 706 (e.g. CO2 laser with a wavelength of λ≈10.6 μm). The infrared radiation is focused by way of a focusing optical unit, passing through an opening 711 present in a collector mirror 710 embodied as an ellipsoid and being guided onto a target material 732 (e.g. tin droplets) which is generated by a target source 735 and supplied to a plasma ignition position 730. The infrared radiation 706 heats the target material 732 situated in the plasma ignition position 730 in such a way that the target material transitions into a plasma state and emits EUV radiation. This EUV radiation is focused by way of the collector mirror 710 onto an intermediate focus IF and enters through the latter into a downstream illumination device, the edge 740 of which is indicated merely schematically and which has a free opening 741 for the light entrance.
What is of substantial importance for the dose stability or time stability of the EUV emission characteristic achievable in an EUV light source or laser plasma source and for the realizable EUV luminous efficiency is that the tin droplets “flying into” the laser plasma source very quickly (e.g. with an injection rate in the region of 100 kHz or with a time interval of e.g. 10 μs) with increasing light requirements are hit individually in a highly precise (e.g. with an accuracy of more than 1 μm) and reproducible manner by the laser beam atomizing the droplet. In the aforementioned setup, this in turn requires highly accurate setting of the droplet position and highly accurate tracking of the infrared radiation 706 generated by e.g. the CO2 laser.
Both the droplet position and the focal position of the laser beams to be tracked accordingly can be determined using a so-called beam propagation camera, wherein both the laser beams in the “forward direction” (i.e. the infrared radiation 706 prior to incidence on the respective target droplets) and the laser beams in the “backward direction” (i.e. the infrared radiation 706 reflected back from the respective target droplet) are detected and the measurement data required for the laser beam guidance and droplet guidance are obtained.
The problem occurring here in practice is that, inter alia, the infrared radiation 706 reflected back from the target droplets has a comparatively weak intensity and this makes an exact metrological detection of the droplet position, and hence also the highly accurate tracking of the infrared radiation 706 generated by the CO2 laser, more difficult. With regard to the prior art, reference is made by way of example to U.S. Pat. No. 8,237,922 B2 and U.S. Pat. No. 5,329,350.
FIG. 13 serves for elucidating one possible conventional approach for light beam analysis. In this case, the light beam to be analyzed is focused by a focusing lens element 10 onto a four-quadrant sensor 20 arranged in the image-side focal plane thereof and composed of four sensors 21-24 which measure the light intensity, wherein the position of the light beam is determined from computation of the light intensities measured by the four sensors 21-24.
Here, however, in the above-described application of analyzing for instance the infrared radiation in an EUV light source or laser plasma source, in practice the problem occurs that the light beam to be measured is subjected to great variations, wherein in particular the divergence of the light beam in the case of a defocus of the light beam or laser beam with respect to the target droplet and also the direction of the light beam (corresponding to a “pointing” of the beam) change and wherein a lateral displacement of the beam additionally occurs as well.